1. Field of the Invention
The present invention generally relates to the field of optoelectronics, and more specifically to an isolated pixel photoconductive structure for voltage modulating a liquid crystal layer in a liquid crystal light valve.
2. Description of the Related Art
The silicon photoconductor based liquid crystal light valve (LCLV), or spatial light modulator, performs the function of converting an input light image having a certain wavelength, intensity, and coherence conditions into an output image in which some or all of these parameters are varied. Applications of LCLVs include image amplifiers, wavelength converters, incoherent-to-coherent image converters, and adaptive optics. While image amplifiers find uses in large screen displays such as theaters, flight simulators, and command and control displays, image wavelength converters are used for displaying visible images from infrared scenery and the like. Incoherent-to-coherent image converters are used primarily for optical image processing.
The silicon LCLV to which the present invention constitutes a novel improvement is described in an article by U. Efron et al, entitled "The silicon liquid-crystal light valve", J. Appl. Phys. 57(4), Feb. 1985. The device consists of a high resistivity, .pi.-silicon photoconductive layer or substrate, coupled with a dielectric silicon dioxide layer to form an MOS structure. A unified thin-film structure consisting of a dielectric mirror and a light blocking layer provides the high broadband reflectivity required, as well as optical isolation of the photoconductor from the high-intensity readout beam. The readout beam is reflected by the dielectric mirror through the liquid crystal. The latter is usually operated in a hybrid field effect mode. The MOS mode of operation consists of periodic depletion and accumulation phases. In the depletion (active) phase, the high-resistivity .pi.-silicon is depleted completely, and electron-hole pairs generated by the input light are swept by the electric field, thereby producing the signal current that activates the liquid crystal.
The electric field existing in the depletion region acts to focus the signal charges, and to preserve the spatial resolution of the input image at relatively low photogenerated charge densities. However, at larger charge densities associated with a larger signal dynamic range, the photogenerated charge carriers will diffuse or spread laterally in a potential free region near the silicon back surface where the charge is generated. This is caused by partial collapse of the depletion region as the charge density increases. The signal charge goes through further lateral drift and diffusion as it drifts through the silicon thickness, and most importantly, to an even greater extent at the silicon/dielectric layer interface where the charge resides for a finite length of time. This lateral drift and diffusion of the signal charge results in significant loss in device resolution.
The lateral spread of photogenerated charge at the silicon/dielectric interface has been conventionally limited by means of a grid of "microdiodes" formed by regions of opposite doping polarity implanted into the silicon layer at the interface. The grid acts to focus the incoming charge carriers into the resolution cell defined by it, as well as to form "charge buckets" of carriers already residing at the interface.
Although effective at relatively low levels of photogenerated charge, the microdiode grid cannot contain the signal charge residing at the interface at high excitation levels. Once the potential wells formed by these diodes are partially filled with the signal charge, the surface potential is decreased and the charge can spill over to adjacent pixel areas. In addition, the microdiodes are not operative to prevent lateral spread of charge carriers at the silicon back interface at which the charges are generated, or in the bulk portion of the silicon layer. This limits the liquid crystal voltage swing and thereby the signal dynamic range and image contrast attainable in a LCLV without an accompanying loss of resolution.